1) Field of the Invention
The present invention relates to adhesive tapes for electronic parts to be used for adhering between parts around leadframes making up a semiconductor device, e.g., lead pins, semiconductor chip mounted substrates, heat spreader, semiconductors themselves.
2) Description of the Related Art
Conventionally, adhesive tapes for fixing a leadframe, TAB tapes, etc., are used as adhesive tapes for use in the interior of resin-molded type semiconductor devices. For example, the adhesive tapes for fixing a leadframe have been used to fix lead pins of the leadframe in order to enhance the efficiency for producing the leadframe itself and the whole semiconductor assembly stages. In general, a leadframe manufacturer tapes a leadframe, and brings it to a semiconductor manufacturer, at which a semiconductor chip is mounted thereon followed by molding the leadframe with a resin. For this reason, it has been required for the adhesive tapes for fixing the leadframe not only to possess general reliability in a level required for semiconductor and enough processability at the time of taping, but also to possess sufficient adhesive strength at room temperature immediately after taping and heat resistance enough for withstanding thermal process at the stages of assembling semiconductor devices.
Conventionally, the adhesive tapes for use in such an application include adhesive tapes applying on a support film of a polyimide film, etc., an adhesive comprising polyacrylonitrile, polyacrylate or a synthetic rubber resin such as acrylonitrile-butadiene copolymer solely, or modified with any other resin or blended with any other resin to come into a B-stage.
In recent years, resin-molded type semiconductor devices as shown in FIGS. 1 to 3 have been developed or produced. In FIG. 1, the device has a construction in which lead pins 3 and plane 2 are connected by means of an adhesive layer 6, a semiconductor chip 1 is mounted on the plane 2, and together with bonding wires 4 between the semiconductor chip 1 and the lead pins 3, they are molded with a resin 5. In FIG. 2, the device has a construction in which the lead pins 3 on the leadframe are fixed with the semiconductor chip 1 and an adhesive layer 6 and together with a bonding wire 4, they are molded with a resin 5. In FIG. 3, the device has a construction in which a semiconductor chip 1 is mounted on a die pad 7, an electrode 8 is fixed with an adhesive layer 6, the spaces between semiconductor chip 1 and the electrode 8 and between the electrode 8 and lead pins 3 are each connected with bonding wires 4, and they are molded with a resin 5.
In the adhesive layer in the resin-molded type semiconductor devices shown in FIGS. 1 to 3, the use of an adhesive tape to which a conventional adhesive is applied has the problems that the generated gas stains the lead to cause deterioration of the adhesive strength or causes generation of package crack, because of insufficient heat resistance. It is therefore required to develop adhesives for electronic parts having sufficient heat resistance and reliability and adhesive tapes for electronic parts using them.
The present inventor has solved before the above problems by inventing adhesive tapes using an adhesive containing a polyimide composed of repeating units represented by the below-described formulas (1a) and (2b). (Japanese Patent Application Laid-open Nos. 325533/1996 and 67559/1997).
These adhesive tapes however have various problems. For example, in the semiconductor device having a structure shown in FIG. 2, a punched adhesive tape is adhered to lead pins 3 and then the semiconductor chip is allowed to adhere thereto. In such a case, since an adhesive having the same glass transition temperature is used on the other side of the tape, there is a problem that the adhesive on the side of the lead pins 3 softens at adhesion temperature of the semiconductor chip 1 to cause distortion or movement of the lead pins 3, and consequently the products can not be used because of shorting. The adhesion to the lead pins 3 is capable of carrying out by heating the lead pins 3 at a temperature of 80.degree. C. higher than the glass transition temperature, but there is the possibility of damaging the semiconductor chip, because the adhesion requires much time or pressure under heating the lead pins 3 to the above-mentioned temperature. The same problem is caused on adhesive tapes composed of a monolayer of thermoplastic polyimide. Furthermore, in case of using the monolayer adhesive tapes, processing thereof becomes difficult, because the distortion of the lead pins by thermal expansion is easily caused in the heating stage because of not having the supporting film.
In the semiconductor device shown in FIG. 1 or FIG. 3 in which the semi-conductor chip is not directly bonded to the lead pins, there are two stages of bonding with heat under pressure. Therefore, there is a problem that the distortion or movement of the lead pins is caused because of softening of the adhesive when the second bonding stage is carried out at the same temperature as in the first bonding stage.
Japanese Patent Application Laid-open No. 291236/1994 discloses a method of bonding heat radiator to the lead frame in the semiconductor device with an adhesive tape comprising polyimide adhesive layers having each a different glass transition temperature (Tg). However, there is no detailed description concerning components of the adhesive to be used. Generally, resins which have the same Tg show different behavior one another under a higher temperature than Tg, when the components or molecular weights thereof are different one another, even though having the same Tg. Accordingly, sufficient reliability is by no means obtainable in the adhesive for electronic parts, when the adhesive tape is produced by merely laminating resins having each a different Tg. Japanese Patent Application Laid-open No. 291152/1994 discloses the same adhesive layer as described above. However, there is no detailed description about the components of the adhesive to be used, but only discloses that combinations of resins such as polyimide, polyetherimide, polyetheramide imide, etc. can be used. However, such combinations have the problem that they easily cause interlayer separation by accumulation of strains suffered in the production stages for electronic parts, because adhesive properties of such combination of resins having each a different molecular structure are not always excellent.
The present invention has been made for the purpose of solving such problems in the prior arts. Namely, an object of the present invention is to provide adhesive tapes for electronic parts capable of adhesion at a relatively low temperature, with keeping electrical insulation and having sufficient reliability.